Untightening of Nuts & Bolts

Corroded NutIf a nut is untightened immediately after being tightened, the torque needed to untighten it will be less than that needed to tighten it. This is due to the portion of the torque which actually stretches the bolt always acting in the untightening direction. Hence when a nut is immediately untightened, usually about 20% less torque is needed than was needed to tighten it in the first place.

As the time passes from when the nut was tightened, the torque needed to untighten it tends to increase. After half a day or so, the release torque can be typically up 10% greater than the tightening torque. This is due to a number of effects including embedding of the contact surfaces and changes in temperature affecting the friction conditions. Such effects typically increase the friction and hence a greater release torque is required. Because of such friction changes, the use of torque auditing methods (measuring the backoff - or crack-off torque or the tightening or crack-on torque) to assess if the bolt being tightened correctly is only accurate if it is completed shortly after the assembly was tightened.

>The greater the changes in the temperature and environment experienced by a bolted joint, the more rapid are the changes in the friction and subsequent change to the release torque. In subsea applications, in applications sustaining elevated or low temperatures, significant changes in the friction conditions can rapidly occur. The properties of any lubricant will change over time, which in some applications, can lead to galling of the surfaces resulting in a dramatic increase in the release torque i.e. by as much as 50% to 100% of the tightening torque.

Over a prolonged time period, oxidation and corrosion of the thread interfaces and nut face can occur making removal of the nuts problematic without destroying the bolt. In such circumstances nut splitters and similar measures are needed for nut removal.

Joint containing liquid nitrogen When hydraulic torque tightening tools are used, problems can occur in that the tool used to tighten the nut cannot remove it. Hence it is normal practice for the tool to be selected based upon 80% of its rated torque value to allow some margin for subsequent nut removal. Even with such an allowance, a larger rated tool may be needed to remove the nut under certain circumstances.

To assist in preventing problems, avoid having too much thread protrusion past the top of the nut (more than two thread pitches unless hydraulic tensioners are used). Also consider the use of protective caps that pass over the thread/nut to protect the surfaces from the effects of corrosion and incidental damage.